The present invention relates to a three-axis accelerometer, and more particularly, to a circuit for correcting acceleration values for X-axis, Y-axis, and Z-axis directions detected by a three-axis accelerometer.
FIG. 1A is a schematic circuit diagram of a conventional sensor unit 100 for correcting the output of a three-axis accelerometer 1. The three-axis accelerometer 1 is a capacitive type accelerometer employing MEMS (Micro Electron Mechanical Systems) technology. The three-axis accelerometer 1 detects a plurality of acceleration values as capacitance values for orthogonal axes, that is, the X-axis, Y-axis, and Z-axis directions.
The sensor unit 100 includes a capacitance-voltage (C-V) conversion circuit 110, temperature coefficient of offset (TCO) circuits 121 to 123, and operational amplifiers 131 to 133. The C-V conversion circuit 110 is connected to the three-axis accelerometer 1. The operational amplifiers 131 to 133 are connected to the TCO circuits 121 to 123, respectively. The C-V conversion circuit 110 receives an acceleration detection signal (capacitance value) from the accelerometer 1 and converts the capacitance value to a voltage value to generate an input signal Vin, which is shown in FIG. 1B.
The TCO circuits 121 to 123 generate temperature coefficient values TCx, TCy, and TCz for correcting the temperature dependency of the three-axis accelerometer 1, that is, the temperature dependency of acceleration values Ax, Ay, and Az for the X-axis, Y-axis, and Z-axis directions that are detected by the accelerometer 1. The three TCO circuits 121 to 123 have the same structure. Thus, the structure of only the TCO circuit 121 is described below.
The TCO circuit 121 receives a trimming signal Tx for setting the temperature coefficient value TCx. The TCO circuit 121 includes a decoder for receiving the trimming signal Tx and a register for storing the temperature coefficient value TCx. The TCO circuit 121 decodes the trimming signal Tx with the decoder and reads the temperature coefficient value TCx from the register based on the decoding result. The temperature coefficient value TCx read from the TCO circuit 121 is provided to the operational amplifier 131 as a reference voltage. In the same manner, the TCO circuit 122 decodes a trimming signal Ty, reads the temperature coefficient value TCy, and provides the temperature coefficient value TCy to the operational amplifier 132. Similarly, the TCO circuit 123 decodes a trimming signal Tz, reads the temperature coefficient value TCz, and provides the temperature coefficient value TCz to the operational amplifier 133.
The operational amplifiers 131 to 133 each have a first input terminal, a second input terminal, and an output terminal. An input capacitor 141 for holding the input signal Vin is connected between the first input terminal of each of the operational amplifiers 131 to 133 and the C-V conversion circuit 110. The second input terminal of the operational amplifier 131 is provided with the temperature coefficient value TCx generated by the TCO circuit 121. The second input terminal of the operational amplifier 132 is provided with the temperature coefficient value TCy generated by the TCO circuit 122. The second input terminal of the operational amplifier 133 is provided with the temperature coefficient TCz generated by the TCO circuit 123. A feedback capacitor 151 is connected between the first input terminal and the output terminal of the operational amplifier 131. A feedback capacitor 152 is connected between the first input terminal and the output terminal of the operational amplifier 132. A feedback capacitor 153 is connected between the first input terminal and the output terminal of the operational amplifier 133.
The operational amplifier 131 calculates the difference between the input signal Vin (acceleration value Ax) and the temperature coefficient value TCx (reference voltage). Then, the operational amplifier 131 amplifies the difference to generate an acceleration signal Xout. More specifically, the operational amplifier 131 corrects the acceleration value Ax with the temperature coefficient value TCx to generate the acceleration signal Xout for the X-axis direction. In the same manner, the operational amplifier 132 corrects the acceleration value Ay with the temperature coefficient value TCy to generate an acceleration signal Yout for the Y-axis direction. The operational amplifier 133 corrects the acceleration value Az with the temperature coefficient value TCz to generate an acceleration signal Zout for the Z-axis direction.
As described above, the conventional sensor unit 100 corrects the acceleration values for the X-axis, Y-axis, and Z-axis directions with the corresponding temperature coefficient values TCx, TCy, and TCz provided separately from the three TCO circuits 121 to 123 to compensate for the offset temperature characteristics of the accelerometer 1.
The conventional sensor unit 100 needs the three TCO circuits 121 to 123 to separately obtain the temperature coefficient values TCx, TCy, and TCz. As described above, the TCO circuits 121 to 123 each include a decoder and a register. Thus, each TCO circuit occupies a relatively large circuit area. As a result, the three TCO circuits 121 to 123 occupy an extremely large circuit area provided on the chip of the sensor unit 100. This increases the chip size of an ASIC (application-specific integrated circuit) on which the sensor unit 100 is mounted and ultimately increases the chip cost.
Recent accelerometers are required to be fabricated at a lower cost. One way to lower the cost is to reduce the chip size of the sensor unit. Thus, it would be advantageous to have a smaller sensor unit.